1. Field of the Invention
The invention relates to a reluctance motor with a stator which has a three-phase current stator winding for generating a rotary magnetic field and a rotor which is located on a shaft and which is made primarily of a ferromagnetic material. In particular, the invention relates to such a reluctance motor in which the rotor has a set number of angular regions of the same peripheral angle which adjoin one another in the peripheral direction, preferably having at least one pair of flux guidance regions facing the stator, with flux guidance properties which differ in the main direction of the rotary field.
2. Description of Related Art
Reluctance motors are known from the prior art (compare, for example, Lueger, Lexikon der Technik, vol. 14, Lexikon der Feinwerktechnik, page 315) as independently accelerating synchronous motors. The stator of a conventional reluctance motor is no different from the stator of a conventional synchronous or induction motor which is conventionally operated with three-phase current, as is the conventional reluctance motor. The three-phase current stator winding is designed on a standard basis such that the center plane of each of the coils assigned to one of the three phases lies on the axis of the reluctance motor. In a conventional reluctance motor, as in a synchronous or induction motor, windings with a pole number p greater than two and a number of holes q greater than 1 are allowable.
Typically the three-phase current stator winding of a conventional reluctance motor is made with 4 poles with coils being assigned to each of the three phases and with the coils being distributed in the slots over the entire periphery of the stator; see, for example, S. A. Naser, Electromechanics and Electric Machines, John Wiley and Sons, Inc. 1979.
Accordingly, the rotor of the conventional reluctance motor, in four angular regions of the same peripheral angle which adjoin one another in the peripheral direction, has a pair of flux guidance regions facing the stator with flux guidance properties which differ in the main direction of the rotary field. In the conventional reluctance motor, the pairs of flux guidance regions facing the stator are formed with flux guidance properties which differ in the main direction of the rotary field in angular regions of 90xc2x0 each by the rotor in half of the angular regions, therefore over an angle of 45xc2x0, being countersunk. Since the main direction of the rotary field in a block rotor always points in the radial direction of the rotor, the countersinking in the rotor results in flux guidance regions with different magnetic resistances, and therefore, different flux guidance properties.
Conventionally, the rotor of a conventional reluctance motor is made as a squirrel-cage rotor. Therefore, in the operation of a conventional reluctance motor, two torques take effect. In the acceleration range, the conventional reluctance motor develops an asynchronous torque, as a result of the widening of the air gap due to the countersinking in the rotor, the characteristics deteriorate compared to an undamaged rotor of an induction motor. At synchronous rpm, a synchronous so-called reluctance or reaction torque is formed because the rotor, which turns synchronously with the rotary field, tries to assume a position in which the magnetic energy in the air gap is smallest. When the motor is loaded, the rotor would like to remain in this position, it must however lag by a small spacial angle (load angle). The highest torque occurs at a load angle of 90xc2x0/p and is called the pull-out torque. Conversely the transition from the asynchronous characteristic to synchronism takes place suddenly as a synchronization process. Whether this dynamic pulling into synchronism is possible depends on the stationary load torque and the moment of inertia to be accelerated.
The existing statements indicate that a conventional reluctance motor runs with a speed of 6000/p rpm. Since, for a plurality of applications, clearly lower rpms are necessary and since the speed of a conventional reluctance motor can be reduced only to a limited degree by increasing the pole number p, to reduce the rpm and/or increase the torque, mechanical gearing and/or electric frequency converters are regularly used. These additional components, on the one hand, increase the production costs of a conventional reluctance motor with low rpm, and on the other hand, adversely affect the efficiency.
One alternative for ensuring low synchronous rpm is represented by the subsynchronous reluctance motor which is, likewise, known from the prior art (compare, for example, Lueger, Lexikon der Technik, vol. 14, Lexikon der Feinwerktechnik, page 315). This subsynchronous reluctance motor is operated single-phase and on its stator in a number of angular regions of the same peripheral angle which adjoin one another in the peripheral direction, a number which corresponds to the number of angular regions on the rotor, has one pair of flux guidance regions facing the rotor with flux guidance properties which differ in the main direction of the rotary field. For the subsynchronous reluctance motor, the flux guidance regions of different flux guidance properties are produced by the countersinking in the stator. As already mentioned, in the subsynchronous reluctance motor, the number of angular regions in the rotor corresponds to the number of angular regions in the stator. The number of angular regions PSL can be chosen independently of the pole number of the stator winding. The rpm of the subsynchronous at reluctance motor is 3000/PSL rpm.
The subsynchronous reluctance motor can be used only to a very limited degree, since it must be started to the synchronous rpm and then develops only a synchronous torque which pulsates with twice the main frequency from zero to a maximum. Accordingly, the pull-in torque of the subsynchronous reluctance motor is very small.
In addition to the conventional reluctance motor and the subsynchronous reluctance motor, the electronically switched reluctance motor is known from the prior art (compare, for example, Encyclopaedia Britannica CD 97, xe2x80x9cEnergy Conversionxe2x80x9d, xe2x80x9cReluctance Motorsxe2x80x9d). This electronically switched reluctance motorworks, as the name says, with an electronically switched direct current. The electronically switched direct current magnetizes, at the same, time, two coil windings on the flux guidance areas which are opposite one another in the stator with low magnetic resistance, therefore ferromagnetic poles. The center planes of the two coil windings therefore run tangentially to the block rotor. The numbers of angular regions in the rotor and stator are different in an electronically switched reluctance motor in order to generate a torque which engages the rotor when the direct current is switched from one coil pair to another coil pair. Since the direct current of this type of reluctance motor is electronically switched, theoretically all rpm can be effected for the rotor; of course, here, an electronic control unit is necessary for this purpose. However, such is problematic in the electronically known reluctance motors that only relatively low torques can be transmitted so that additional gearing is necessary to achieve the desired drive torques. In addition, in these motors in the lower rpm range, a rpm fluctuation frequently occurs which is caused by torque fluctuations and which must be corrected by an expensive electronic control.
Proceeding from the aforementioned prior art, the object of the invention is to make available a reluctance motor which is improved especially with reference to the possibilities of step-down and dynamic pull-in.
In particular, it is an object of the present invention to obtain a reluctance motor which is able to step down the rpm of the rotor without the use of gears or electronic control of the rotating field to do so.
According to the first teaching of the invention, the indicated objects are achieved by the stator, in a stipulated number of angular regions of the same peripheral angle which adjoin one another in the peripheral direction, preferably, having at least one pair of flux guidance regions facing the rotor with flux guidance properties which differ in the main direction of the rotary field, and by the number of angular regions on the stator differing from the number of angular regions on the rotor by an integral multiple of the pole number, preferably the simple pole number, of the three phase current stator winding.
The configuration of the reluctance motor according to a first aspect of the invention ensures stepping down of the rpm of the rotor as compared to the rpm of the rotating field with a simultaneous increase of the nominal torque of the reluctance motor. The synchronous rpm of the reluctance motor configured according to the first aspect of the invention is found as follows:
nx=fxc2x7120([w2xe2x88x92w1]/[pxc2x7w2])
where
nx=synchronous speed in rpm
f=frequency of the three-phase current
w1=number of angular regions on the stator
w2=number of angular regions on the rotor
p=number of poles of the three-phase current stator winding
The reluctance motor according to this first aspect of the invention easily enables reduction ratios in the range of 1:20 and more, when the torque increases, for example, by a factor of 3 to 5. The reluctance motor according to this first aspect of the invention, therefore, has the properties of a geared motor without gearing being necessary. It allows transmission of large torques with a small structural size, operates with high efficiency, and can be produced with low production engineering cost. The reluctance motor according to the first teaching of the invention has good synchronism properties, and at rest, has a high holding torque when direct current magnetizes the stator winding.
With the reluctance motor according to the first aspect of the invention, as already mentioned, as compared to conventional electric motors, very high torques can be achieved, the motor weight being comparatively small. These high torques are possible by the motor having arrangements with a total of three different xe2x80x9cpole numbersxe2x80x9d, a large number of the existing magnet poles at the same time being magnetically engaged to the magnetic flux guide. Thus, in many cases, a gear reducer can be eliminated since the motors also deliver very high torques even in the range of low rpm. Other important advantages of the reluctance motors according to the first aspect of the invention arise by their having, in comparison with conventional motors, very good smooth running properties in low rpm ranges, and no slippage, regardless of the load and voltage fluctuations. The reluctance motor according to the first teaching of the invention can be operated on the grid and by means of commercial frequency converters (even without return). The motor current changes only little when the motor is loaded or blocked so that the motor cannot be destroyed either in case of overload or in the blocked case.
If the flux guidance regions of different flux guidance properties are formed alternately by air and the ferromagnetic material of the stators and/or rotors, i.e. the flux guidance regions of low magnetic resistance consist of xe2x80x9cteethxe2x80x9d of the ferromagnetic material of the stator and/or rotors and the flux guidance regions with high magnetic resistance consist of air gaps between the xe2x80x9cteethxe2x80x9d of the stators and/or rotors, both production engineering costs and also the costs for production of flux guidance regions of different flux guidance properties are very low.
A further reduction of production engineering costs is ensured by the fact that, for the case in which the flux guidance regions of low magnetic resistance are formed from the ferromagnetic material of the stator, the number of angular regions of the stator corresponds to the number of slots of the three-phase current stator winding. In this case, the flux guidance regions of low magnetic resistance can be formed directly as an elongation of the flux guidance regions of low magnetic resistance which are present between the slots anyway.
In practice, it has been shown that the reluctance motor according to the first aspect of the invention has especially good properties with respect to the rated torque and synchronizing properties, if the three-phase current stator winding is made with 2 or 4 poles.
Furthermore, it has been found in practice that the properties of the reluctance motor according to the first aspect of the invention are improved with respect to the rated torque and synchronizing properties for the case in which the number of angular regions on the stator and rotor is clearly greater, preferably by a factor of at least 5, than the pole number of the three-phase current stator winding.
To increase the rated torque, it is furthermore advantageous if the widths of the flux guidance regions with different flux guidance properties on the component with the highest number of angular regions essentially agree, and at the same time, the widths of the flux guidance regions formed by ferromagnetic material on the remaining components corresponds to the widths on the component with the largest number of angular regions.
If the stator and rotor each have at least one other layer of the flux guidance region pairs with flux guidance properties which differ alternately in the main direction of the rotary field and the layers of the stators and rotors follow one another in alternation, a clear increase of the rated torque of the reluctance motor according to the first aspect of the invention is ensured. This can be easily substantiated by the fact that the magnetic forces for the described configuration of the reluctance motor according to the first aspect of the invention are applied to twice the number of flux guidance elements with low magnetic resistance.
An optimum ratio between the rated torque and material cost for the flux guidance regions is obtained when, on the one hand, it is ensured that the flux guidance regions of one layer of the stator or rotor, which are made of a ferromagnetic material and which lie between two other layers of the stator or rotor, are roughly as high as wide in the main direction of the rotary field, and/or on the other hand, it is ensured that the flux guidance regions of the stator or rotor which are located in the immediate vicinity of the return elements of the stators or rotors and which are made of ferromagnetic material are roughly half as high as wide in the main direction of the rotary field.
Since the forces which tend to keep the magnetic energy low in the air gap of the reluctance motor are applied to the flux guidance regions of low magnetic resistance of the rotor, for the same size, it is advantageous to form the rotor as an external rotor since, in this case, the attacking forces apply a higher torque as a result of the better lever ratio.
As alternative to the configuration of the flux guidance regions of different flux guidance properties by air and ferromagnetic material, it is possible to form the flux guidance regions of different flux guidance properties by permanent magnets which are located either on the stator or rotor and which are polarized oppositely in the main direction of the rotary field. This configuration of the flux guidance regions, for otherwise the same geometrical design, increases the rated-load torque of the reluctance motor according to the first aspect of the invention, but at the same time causes higher production costs.
A reluctance motor according to the first aspect of the invention in which the flux guidance regions of different flux guidance properties are formed either on the stator or on the rotor by permanent magnets which are polarized oppositely in the main direction of the rotary field, with respect to its rated-load torque and its synchronizing properties, behaves optimally when the difference of the number of angular regions on the stator and rotor corresponds to an integral multiple of the number of pole pairs, preferably, the simple pole pair number, of the three-phase current stator winding. In practice, this means that a four-pole reluctance motor according to the first aspect of the invention in which there are permanent magnets on the stator or rotor behaves optimally when the difference of the number of angular regions on the stator and rotor is two. With this difference, a 4-pole reluctance motor according to the first aspect of the invention, in which the flux guidance regions of different flux guidance properties on the stator and rotor are formed by air and ferromagnetic material, is not serviceable.
A reluctance motor according to the first aspect of the invention in which the flux guidance regions of different flux guidance properties arc formed by permanent magnets which are located either on the stator or on the rotor and which are poled oppositely in the main direction of the rotary field behaves differently also with respect to the optimum ratio of rated-load torque and material cost. An optimum ratio between the rated-load torque and material cost is ensured in a reluctance motor configured in this way by the ferromagnetic flux guidance regions which are located in the immediate vicinity of the return elements of the stators or rotors, which are assigned the flux guidance regions being formed by permanent magnets on the rotor or stator which are polarized oppositely in the main direction of the rotary field being roughly as high as wide in the main direction of the rotary field.
The reduction ratios which can be achieved with the single-stage gear reduction described so far are, in fact, upwardly limited by the fact that the optionally large numbers of angular regions on the stators and rotors, on the one hand, cannot be easily accomplished, and on the other, become problematic with respect to the size of the rated-load torque. Higher reduction ratios are accordingly made available according to another arrangement of the first aspect of the invention by a reduction rotor floating on the shaft being located between the stator and rotor, by the reduction rotor on its surface facing the stator in a preset number of angular regions of the same peripheral angle which adjoin one another in the peripheral direction, each having preferably a pair of flux guidance regions with flux guidance properties which differ in the main direction of the rotary field, by the reduction rotor on its surface facing the rotor in a stipulated number of angular regions of the same peripheral angle which adjoin one another in the peripheral direction, each having preferably a pair of flux guidance regions with flux guidance properties which differ in the main direction of the rotary field, and by the difference of the number of angular regions on the stator and the number of angular regions on the surface of the reduction rotor facing the stator corresponding to the difference of the number of angular regions on the surface of the reduction rotor facing the rotor and the number of angular regions on the rotor and to an integral multiple of the pole number, preferably the simple pole number, of the three-phase current stator winding. The reduction rotor configured as above ensures that, in spite of high frequencies of the stator current, the reluctance motor turns very slowly and uniformly. The described configuration furthermore ensures that the rotor joined to the shaft has a very low effective moment of inertia.
According to a second aspect of the invention, the above objects are achieved by the rotor encompassing the flux guidance regions and the connecting elements for connection to the shaft and by there being a flux guidance rotor which floats on the shaft and which consists of ferromagnetic material for return of the lines of force of the rotary field. This second aspect of the invention can be accomplished alternatively or cumulatively, i.e., separately or in conjunction, with the first aspect of the invention.
The moment of inertia of the rotor can be significantly reduced by the separation of the functions of accommodating the torque and return of the lines of force which is provided according to the second aspect of the invention. Thus, the dynamic pulling of the reluctance motor into synchronism is facilitated. In steady-state operation, the flux guidance rotor then assumes a speed in the vicinity of the synchronous rpm of the rotor so that eddy-current losses are reduced.
Since, in particular rotors made as internal rotors have a high moment of inertia, the second aspect of the invention is advantageously arranged such that the internal rotor is made as a hollow cylinder and that, within the internal rotor, there is a ferromagnetic material flux guidance rotor made as a solid cylinder and supported to float on the motor shaft.
An improvement in the asynchronous starting of a reluctance motor according to the invention is ensured by the bars of a squirrel-cage damper winding running in recesses in the ferromagnetic material to form the rotor flux guidance regions of air. This measure yields a similar arrangement as is known for conventional squirrel-cage rotors of induction motors.
To reduce eddy-current losses, the flux guidance regions made of ferromagnetic material are advantageously built up from electric steel sheets which are insulated from one another.
A positive effect on the operating and noise behavior of the reluctance motor in accordance with the invention is ensured by the flux guidance regions in the stator and/or in the rotor running being inclined in the direction of rotation. A more uniform behavior of the rated torque is guaranteed by this inclination of the flux guidance regions.
Especially for the case in which the reluctance motor of the invention is to have a short structural length, is it advantageous if the stator and rotor include a radial air gap, the reluctance motor is therefore made as a disk-type rotor.
Finally, another advantageous configuration of the reluctance motor in accordance with the invention is achieved by the provision of a transducer or a resolver on the shaft. Using this transducer or resolver, a control unit controls a frequency converter such that loss of synchronism of the reluctance motor of the invention is prevented under load so that, as a result, for the reluctance motor of the invention, a characteristic similar to that of a de motor results.
In particular, there is now a plurality of possibilities for configuring and developing the reluctance motor as of the invention.
These and further objects, features and advantages of the present invention will become apparent from the following description when taken in connection with the accompanying drawings which, for purposes of illustration only, show several embodiments in accordance with the present invention.